Thread: 98 Grand Prix GTP 3800 II, Troubleshooting Rough Idle and Sluggish Acceleration :(

I have a 98 GTP with 2 problems. The first is the car will not idle by its self, It will start quick and rps go from 400 to 900 and then dies in about 4 seconds. The work around is i have to feather the throttle and slowly bring the rps down and the car stays idling fine. Then i accelerate and let off the gas it drops to normal idle just fine, never dies. The second issue is when accelerating the car is sluggish and bogs down, seems to be surging from power to no power. Might be boost related.... The car runs great sometimes and others its hard to get up to the speed limit, it seems to do this with the engine being cold or warm and you cant make it happen on demand.

There was one check engine light on so i hooked it up to the computer and the error code was P0440 EvaPorative Emission System. The guy there told me that it was more than likely the gas cap that was causing the error code, so i replaced it and cleared the code and it seems to be gone for good. This solved the error code but not the 2 problems mentioned above.

As for what has been done to diagnose\repair this issue: New Fuel Pump, Fuel Pressure Regulator, MAF Sensor, IAC Sensor, Plugs, Wires, Air Filter.

Any Ideas\Solutions? I welcome all opinions and would love to get this car running back to the way it should.

Thanks in advance for all that devote their time in helping me resolve these problems.

Ryan

Sounds like a vacuum leak. Check all the lines, especially the one under the snout and see if the T is cracked.

Thanks Fordman77, i will do that first thing in the morning and post the results.

Ok sorry for the late update, i replaced all the vaccum lines and almost all of them were cracked and leaking. Now the car idles fine only when its cold. As for the acceleration its not surging it just feels like its alot slower acceleration. Im thinking about changing the throttle position sensor and see what that does. Still no engine codes. Although the srs airbag comes on and flashes then goes away. Oh i hooked a vaccum guage to the intake manifold and the reading was a steady 15. Not sure if thats normal. The idle is at 700 rpm. Any thaughts?

What happens after it warms up? You said it idles fine cold. Not sure for the GTP's, but that idle sounds a bit low. Is that fully warmed up in Park, or what?

The idle is 700 when its cold or warm in park. I just ordered an obd 2 reader to get all the sensor information and see what exactly is going on. I should have it in a week and hope that will help in figuring out whats wrong. Do you know what the vaccum should be?

Sorry the idle will not hold whrn engine is warm, i have to feather the throttle and slowly bring the rpms down to 700 for it to stay idling. Although when driving i will not die after letting off the gas, it is slow on accelerating but not surging anymore.

I know you said you replaced the MAF, but have you tried unplugging it and seeing if it runs any better? It may idle bad at first and need some throttle to stay running but should eventually run on its own. Just take it for an EASY drive (NO BOOST) and see how it runs. It's possible that you may have a bad MAF, either that or it's dirty somehow.

What brand and type (copper, platinum, etc) of plugs did you use? Part # from them?

I put bosh platnium 4 in it, when i checked the vaccum it was at15 during idle in park @ 700 rpm is that normal?

If you are close to stock or have no cam, your Vacuum gauge should read 20-21 HG/in.

I will replace the plugs tomarrow and tripple check all the vaccum lines. I also just replaced the throttle position sensor, the only one i havent replaced is the air temp sensor. The car as of right now is stock. Thanks for all the advice guys. I will post an update with the results tomarrow

Originally Posted by 850Fishing

I will replace the plugs tomarrow and tripple check all the vaccum lines. I also just replaced the throttle position sensor, the only one i havent replaced is the air temp sensor. The car as of right now is stock. Thanks for all the advice guys. I will post an update with the results tomarrow

If you have a compressor, you can get a plumbing end cap drill out the end and put an air hose connector on it. You would need the appropriate size to match your TB. Put about 10-12 pounds of air in it and your vacuum leak will find you. If you hear it but can't see it, try a little soap and water or just water. I keep an empty spray bottle in the garage just for odd things.

Or while the car is running at idle you car spray carb or brake cleaner around spots where vacuum could leak (sealing surfaces, lines, etc..) and see if the motor changes tone at all. Usually when you do this and hit a leak the engine will speed up and smooth out for a second until the leak returns. Been doing it this way for years and it always works.

Cool idea on the TB/air thing. I'll have to try that sometime and see how well it works.

Here is some info on the plugs you can use for your car...

4) Spark Plugs - Change out those stock spark plugs with new ones to restore any lost power. Brands that most WBody owners tend to go with are either Copper Core Autolites or NGK Spark Plugs. Get these at your local auto parts store also. Others can chime in if they use something else. They range as follows in order from hottest to coldest plugs...remember...you want the hottest spark possible...so running a super cold plug on a larger pulley to combat knock isn't the best plan...ACD's, AL606's, and AL605's are basically for stock cars or very small bolt ons still running the stock S/C Pulley or modded N/A guys...you want to keep a hot spark. NGKTR55/AL104's for your 3.4" Setup, TR6's are a tad cooler...best used around the 3.25" pulley area, and AL103's...are for the extreme supercharged application...3.0" pulley or lower. Here is a picture to reference the firing order and the correct coil number if yours aren't numbered or you just need a quick reference picture: http://i33.photobucket.com/albums/d6.../plugwires.jpg

AC Delco Factory Replacements: Stock
Autolite 606: Stock
Autolite 605: 1 Heat Range Cooler
NGK TR55: 1 Heat Range Cooler
Autolite 104: 2 Heat Ranges Cooler
NGK TR6: 2 Heat Ranges Cooler
Autolite 103: 3 Heat Rangles Cooler

Ok, got autolite 606 plugs gaped them at .060 took the car for test drive and runs good untill boost then sputters/misfires. Pulled the plugs out droped the gap to .050 and its alot better but stull missing on boost, about to take them out again and drop the gap to .040 and see what that does. Tge car seems to atart and hold the idle good when its hot now so thats a good thing. Will post update soon.

Ok gaped the plugs to .040 and all the misfire and sputtering are gone boost seems ok car is running good. Acceleration feels about 75% when boost hits. Should i go with hotter plugs or adjust the gap again?

I will do the carb cleaner on all the vac line fittings and see if i have a leak. I will post the results after im finished.

Ok thanks for the heads up on that would of killed me self, lol. I will do this right after i get off wotk to day.

Amy thaughts on the spark plug gap working good at .040 whrn it should be at .060